The morphogenesis of a unique amebal virus of Entamoeba histolytica (Rahman strain) has been studied by electron microscopy. Arrays of parallel packed beaded viruses 300 nm long assemble in the nucleoplasm. The virus isolated in CsC1 gradients exists as a monomer of 14 beads or sometimes as a dimer of 28 beads and 600 nm long. Two animal models for studying the virulence of E. histolytica have been developed. Brains of newborn mice are susceptible to amebae inoculated intracerebrally. Amebae produce a fatal invasive disease with multiple abscesses. The newborn hamster liver is extremely susceptible to E. histolytica, which can be inoculated intrahepatically through the abdominal wall without laparotomy. Progress of disease can also be followed by observing the liver in situ. The disease consists of typical amebic abscesses containing trophozoites. Embryonated eggs and mammalian tissue cultured cells have been found less promising than newborn animals in studying virulence of E. histolytica. Avirulent amebae have been rendered moderately virulent by establishing a culture of survivors of a lytic virus infection. We do not know whether this represents simply the selection of both a virus resistant and virulent population or whether there is a more complex interaction of virus and amebae.